1. Field of the Invention
The present invention relates to an image sensor, more particularly a Complementary Metal Oxide Semiconductor (CMOS) image sensor with anti-saturation function in pixel level which can prevent saturation without degrading color reproduction of an overall image by varying a dynamic range at a per-pixel level.
2. Description of the Related Art
In general, each part of subjects present in the natural world differs in brightness and wavelengths of light. An image sensor is a device that converts different brightness and wavelengths of the subjects into an electrical value of a signal processable level, using photo-reactive properties of semiconductors.
Typically, the image sensor is used at a per-pixel level. A plurality of image sensors are aligned on a line of a certain standard to produce a pixel array. Then images of a certain standard are picked up via the pixel array.
The aforesaid image sensor includes a photo-reactive semiconductor device, and a plurality of transistors for outputting an electrical change of the semiconductor device as an electrical signal of a certain level.
FIG. 4 is a circuit diagram illustrating a 3 TR CMOS image sensor out of image sensors used at a per-pixel level according to the prior art.
Referring to FIG. 4, the image sensor includes a photo diode PD for changing a capacity value in response to light, a reset transistor Q2 for resetting, the photo diode PD to detect a next signal, a drive transistor Q4 for acting as a source follower via an electrical signal stored in the photo diode PD and a select transistor Q5 for selecting an output of a detected value.
That is, if the reset transistor Q2 stays on for a predetermined duration in response to a reset signal Rx, current is stored in the photo diode at an amount proportionate to the capacity value corresponding to light. In addition, the drive transistor Q4 amplifies a voltage of the photo diode PD into the electrical signal within a set range. A detected signal Vout outputted from the drive transistor Q4 is outputted in the addressing order of the pixel array if the select transistor Q5 is turned on.
FIG. 5 is a circuit diagram illustrating a 4 TR image sensor according to the prior art. As shown in FIG. 5, the 4 TR image sensor further includes a transfer transistor Q1 for transferring an electrical signal stored in a photo diode PD in response to a transfer signal Tx, and a floating diffusion FD for receiving charges accumulated in the photo diode via the transfer transistor Q1. At this time, the charges stored in the floating diffusion FD drive the drive transistor Q4.
In the 4 TR image sensor, with a reset signal Rx applied, the photo diode PD generates and accumulates charges in accordance with a received light amount. Then, the transfer transistor Q1 stays on for a predetermined duration and the charges of the photo diode PD are transferred to the floating diffusion FD. The drive transistor Q4 is operated by the charges stored in the floating diffusion FD to generate a detected signal Vout. The detected signal Vout is outputted in the addressing order of a pixel array via the select transistor Q5.
In the image senor as just described, a detected output voltage increases in proportion to the light amount. Due to a limited output range, light incident beyond the dynamic range saturates the image sensor, thus causing a whitening phenomenon in which images are not properly expressed but just whitened.
Typically, the dynamic range DR of the image sensor is defined as a ratio of a minimum measurable light amount Iph—min and a maximum measurable light amount Iph—max as expressed by Equation 1 below:
                    DR        =                  20          ⁢                                          ⁢                      log            ⁡                          (                                                I                  ph_max                                                  I                  ph_min                                            )                                                          Equation        ⁢                                  ⁢        1            
That is, a bigger dynamic range DR ensures concurrent expression of a bright portion and a dark portion within an image Therefore, the dynamic range DR of the image sensor needs to be increased.
But in the image sensor as described above, the dynamic range is fixed by properties of the photo diode PD and/or floating diffusion FD. Thus, light incident beyond the dynamic range renders normal reproduction of images impossible.
Therefore, various studies have been conducted to prevent saturation.
One method has been suggested to solve image reproduction problems caused by saturation. For this purpose, a detected voltage is outputted from a drive transistor logarithmically in response to a light amount received from a photo diode to increase the dynamic range. However, disadvantageously, this distorts colors, deteriorating overall color reproduction and image definition.
In another method to overcome a saturation-induced whitening phenomenon, if saturation occurs during a digital processing, a total output voltage of a pixel array is decreased. In this case, a detected voltage is adjusted even in other portions of an unsaturated image, thus disadvantageously degrading image definition.